Rheumatoid arthritis (RA) is mediated by articular expression of pro-inflammatory cytokines and chemokines. We have discovered that the purine degradation product inosine exerts multiple anti-inflammatory and immunomodulatory effects via combined A2a and A3 agonism, resulting in suppressed expression of TNF-alpha, IL-1 beta, IL-12, MIP-lalpha, and IFN-gamma and enhanced expression of IL-10. We have recently developed a proprietary pro-drug of inosine, 5'-inosine monosulfate (IMS),that is more potent than inosine in vivo and shows dramatic protection against inflammatory injury. In a collagen-induced murine model of autoimmune arthritis, IMS profoundly reduced the incidence and severity of arthritis, reduced TNF-alpha and MIP-lalpha expression, and virtually eliminated neutrophil infiltration and lipid peroxidation. Coupled with the fact that free inosine is an approved OTC nutritional supplement and has an excellent record of safety in man, we envision the introduction of its prodrug IMS as a novel pharmaceutical for the treatment of RA. The Specific Aims of this Fast-Track proposal are: Phase I: Establish that IMS reverses established experimental arthritis and is effective in combination with current clinical anti-arthritic therapies. IMS will be provided to DBA/1J mice rendered arthritic by repeated injection of collagen. Our preliminary data show that introduction of IMS prior to the onset of disease is nearly fully protective. In order to establish the reversibility of established disease, IMS therapy will be introduced at day 35, after the onset of joint disease in this model system. We will also determine whether a one-week pulse of IMS therapy, initiated at day 20, will provide persistent protection against the development of arthritis in the DBA/1J model. Finally, we will establish the synergy of IMS, begun on day 20, with traditional anti-arthritic therapies, including ibuprofen and methotrexate. Progression to Phase II SBIR will require that IMS produces a 50% reduction in the severity of established collagen-induced arthritis and that IMS is synergistic with ibuprofen or methotrexate. In the Phase 2 SBIR, we will determine the pre-clinical pharmacokinetics and biochemical, hematologic, and histopathologic toxicology, and safety pharmacology profile of IMS. The proposed studies will provide the foundation for Phase la and lb clinical safety trials of IMS.